NSAutoreleasePool

The NSAutoreleasePool class is used to support Cocoa’s reference-counted memory management system. An autorelease pool stores objects that are sent a release message when the pool itself is drained.

Important

If you use Automatic Reference Counting (ARC), you cannot use autorelease pools directly. Instead, you use @autoreleasepool blocks. For example, in place of:

NSAutoreleasePool*pool=[[NSAutoreleasePoolalloc]init];

// Code benefitting from a local autorelease pool.

[poolrelease];

you would write:

@autoreleasepool{

// Code benefitting from a local autorelease pool.

}

@autoreleasepool blocks are more efficient than using an instance of NSAutoreleasePool directly; you can also use them even if you do not use ARC.

In a reference-counted environment (as opposed to one which uses garbage collection), an NSAutoreleasePool object contains objects that have received an autorelease message and when drained it sends a release message to each of those objects. Thus, sending autorelease instead of release to an object extends the lifetime of that object at least until the pool itself is drained (it may be longer if the object is subsequently retained). An object can be put into the same pool several times, in which case it receives a release message for each time it was put into the pool.

In a reference counted environment, Cocoa expects there to be an autorelease pool always available. If a pool is not available, autoreleased objects do not get released and you leak memory. In this situation, your program will typically log suitable warning messages.

The Application Kit creates an autorelease pool on the main thread at the beginning of every cycle of the event loop, and drains it at the end, thereby releasing any autoreleased objects generated while processing an event. If you use the Application Kit, you therefore typically don’t have to create your own pools. If your application creates a lot of temporary autoreleased objects within the event loop, however, it may be beneficial to create “local” autorelease pools to help to minimize the peak memory footprint.

You create an NSAutoreleasePool object with the usual alloc and init messages and dispose of it with drain (or release—to understand the difference, see Garbage Collection). Since you cannot retain an autorelease pool (or autorelease it—see retain and autorelease), draining a pool ultimately has the effect of deallocating it. You should always drain an autorelease pool in the same context (invocation of a method or function, or body of a loop) that it was created. See Using Autorelease Pool Blocks for more details.

Each thread (including the main thread) maintains its own stack of NSAutoreleasePool objects (see Threads). As new pools are created, they get added to the top of the stack. When pools are deallocated, they are removed from the stack. Autoreleased objects are placed into the top autorelease pool for the current thread. When a thread terminates, it automatically drains all of the autorelease pools associated with itself.

Threads

If you are making Cocoa calls outside of the Application Kit’s main thread—for example if you create a Foundation-only application or if you detach a thread—you need to create your own autorelease pool.

If your application or thread is long-lived and potentially generates a lot of autoreleased objects, you should periodically drain and create autorelease pools (like the Application Kit does on the main thread); otherwise, autoreleased objects accumulate and your memory footprint grows. If, however, your detached thread does not make Cocoa calls, you do not need to create an autorelease pool.

Note

If you are creating secondary threads using the POSIX thread APIs instead of NSThread objects, you cannot use Cocoa, including NSAutoreleasePool, unless Cocoa is in multithreading mode. Cocoa enters multithreading mode only after detaching its first NSThread object. To use Cocoa on secondary POSIX threads, your application must first detach at least one NSThread object, which can immediately exit. You can test whether Cocoa is in multithreading mode with the NSThread class method isMultiThreaded.

Garbage Collection

In a garbage-collected environment, there is no need for autorelease pools. You may, however, write a framework that is designed to work in both a garbage-collected and reference-counted environment. In this case, you can use autorelease pools to hint to the collector that collection may be appropriate. In a garbage-collected environment, sending a drain message to a pool triggers garbage collection if necessary; release, however, is a no-op. In a reference-counted environment, drain has the same effect as release. Typically, therefore, you should use drain instead of release.

Declaration

Objective-C

- (void)release

Discussion

In a reference-counted environment, since an autorelease pool cannot be retained (see retain), this method causes the receiver to be deallocated. When an autorelease pool is deallocated, it sends a release message to all its autoreleased objects. If an object is added several times to the same pool, when the pool is deallocated it receives a release message for each time it was added.

In a garbage-collected environment, this method is a no-op.

Special Considerations

See Also

In a reference-counted environment, releases and pops the receiver; in a garbage-collected environment, triggers garbage collection if the memory allocated since the last collection is greater than the current threshold.

Declaration

Objective-C

- (void)drain

Discussion

In a reference-counted environment, this method behaves the same as release. Since an autorelease pool cannot be retained (see retain), this therefore causes the receiver to be deallocated. When an autorelease pool is deallocated, it sends a release message to all its autoreleased objects. If an object is added several times to the same pool, when the pool is deallocated it receives a release message for each time it was added.

Special Considerations

In a garbage-collected environment, release is a no-op, so unless you do not want to give the collector a hint it is important to use drain in any code that may be compiled for a garbage-collected environment.

Import Statement

Availability

See Also

Displays the state of the current thread's autorelease pool stack to stderr.

Declaration

Objective-C

+ (void)showPools

Special Considerations

Warning

Unsupported API

This method is part of a module that contains material that is only partially supported—if at all. Do not depend on the existence of this symbol in your code in future releases of this software. Certainly, do not depend on this symbol in production code. The format of any data produced by this method may also change at any time.